In comparison to their inorganic counterpart, the bulk heterojunction (BHJ) polymer solar cells hold many advantages such as low cost of fabrication, ease of processing, flexibility and the potential of preparing large-area devices (Braga et al. Sol. Energy Mater. Sol. Cells, 2008, 92, 418). However, the major draw back is their low power conversion efficiencies (PCEs), which seriously hold back polymer solar cells to be further put into practical applications. Great efforts have been made from various aspects to improve the PCEs during recent years. For instance, nickel oxides and graphene oxides are introduced as candidates for the hole transport layer as well as to prevent electron leakage from the BHJ acceptor to the anode in place of the conventional poly(3,4-ethylenedioxythiophene) (PEDOT) layer. Light reflective material was deposited between the electrode and the photo active layer to drive internal quantum efficiency near quantitative. Slow growth, thermal annealing and mixed solvents are adopted for optimizing the bicontinuous interpenetrating network. In general, polymer's physical properties determine the open-circuit voltage (Voc) and the short-circuit current density (Jsc), and the PCE is defined as Pout/Pin=VocJscFF/Pin. Thus, the research on developing novel semiconducting polymers has aimed at: 1) lowering the bandgap to enhance the Jsc; and 2) lowering the energy level of the highest occupied molecular orbital (HOMO) to improve the Voc.
The recent surge of enthusiasm in bulk heterojunction (BHJ) organic photovoltaics (OPVs) is driven by their potential for fabricating flexible and light-weight solar cells via facile, low-cost solution processing techniques. (Thompson, B. C. et al. Angew. Chem., Int. Ed. 2008, 47, 58-77. Gunes, S. et al. Chem. Rev. 2007, 107, 1324-1338. Spangaard, H. et al. Sol. Energy Mater. Sol. Cells. 2004, 83, 125-146. Hoppe, H. et al. J. Mater. Res. 2004, 19, 1924-1945. Brabec, C. J. et al. Adv. Funct. Mater. 2001, 11, 15-26.) New materials are crucial in order for OPVs to fully mature from research and development into cost effective products. Power conversion efficiency (PCE) of large-area OPV solar cells should be continuously improved. (G. Dennler, G. et al. J. Adv. Mater. 2009, 21, 1323-1338. Scharber, M. et al. Adv. Mater. 2006, 18, 789-794. Coakley, K. M. et al. Chem. Mater. 2004, 16, 4533-4542.) One of the major challenges in design of new polymers is the simultaneous optimization of physical properties, such as morphology and donor-acceptor energy level matching. (Cheng, J.-Y. et al. Chem. Rev. 2009, 109, 5868-5923. Brabec, C. J. et al. J. Chem. Soc. Rev. 2011, 40, 1185-1199. Bundgaard, E. et al. Sol. Energy Mater. Sol. Cells. 2007, 91, 954-985.)
In addition to these parameters, when bandgap, energy levels and morphology were all optimized, the incorporation of a large local dipole moment can also play an important role in charge separation, enhancing the performance of the OPV cells. (Carsten, B. et al. J. Am. Chem. Soc. 2011, 133, 20468-20475.)
There is a need in the art for polymer solar cells that exhibit increased solar conversion efficiency.